Rain has finally quenched the thirst of several drought-stricken states this spring, especially in the Midwest. With that said, the wet weather that has delayed planting and harvesting activities has also presented challenges for spring seeded alfalfa.

In a recent article from University of Minnesota Extension, Craig Schaeffer, Krishona Martinson, and Claire LaCanne note there has been intense, above normal precipitation in the Gopher State so far this season. Even though adequate soil moisture is required for germination, excess rainfall can cause some problems with seedling emergence and stand persistence.

Erosion, flooding, and disease

On highly erodible soil, heavy rain can uproot small seedlings or wash seeds away before they even have the chance to germinate. Medium and heavy textured soils are also subject to crusting when raindrops repeatedly strike the surface. The authors caution crusting problems are compounded when seeding depth is deeper than 1/4 inch.

Flooding threatens to damage and kill new plants by creating an anaerobic environment and inhibiting seedling respiration. Without this gas exchange, ethanol can form in alfalfa roots and restrict biological nitrogen fixation. Young seedlings and plants with only one or two trifoliate leaves are more susceptible to flood damage than plants with multiple stems, especially as temperatures rise.

“Alfalfa seedlings of various stages of development have been reported to be able to tolerate flooding of 10 to 14 days at normal May temperatures, but high temperatures can accelerate damage of flooding,” the authors explain.

With above average rainfall comes saturated soil, which raises the incidence of plant disease like Phytophthora root rot. One way to overcome this is to select resistant varieties and plant alfalfa with seed coatings that contain fungicides.

In addition to weather-related challenges, herbicide residual is a leading cause of poor stand establishment. The authors note an accumulation of dry growing seasons in years past may exacerbate present herbicide residual this year since products need adequate soil moisture and microbial activity to effectively break down.

You might need to reseed

Three to five plants per square foot is a general threshold for 2- to 5-year-old stands, but a spring seeding of alfalfa should have 30 to 35 plants per square foot come fall. By next spring, the authors suggest 15 to 25 plants per square foot is the goal for maximum yield.

To demonstrate the process of natural loss, the authors refer to a study from University of Wisconsin Extension that shows alfalfa seeded at 15 pounds per acre — with 199,000 seeds per pound and 100% pure live seed — had an initial survival rate of 60% after three to four weeks. Another 40% to 50% loss occurred throughout the rest of the year and over the winter.

Assess alfalfa populations in new stands by taking three random plant counts across a field. The authors suggest making a 17-by-17-inch square with PVC pipe or rebar to ensure all measurements have an equal area of 2 square feet.

If new stands have noticeably suffered from erosion or flooding, there are a few options. Starting over by tilling the field and broadcasting or drilling seed is one approach; however, this can exacerbate erosion issues that interfered with establishment in the first place. Therefore, interseeding alfalfa is another solution.

“If thin stands can be thickened by interseeding with no-tillage equipment, it’s cheaper than starting over,” the authors write. “Autotoxicity, or the killing of new seedlings by older plants or their residues, will not be a problem when interseeding spring stands.”

Using a no-till drill is another alternative. To ensure this reseeding is successful, confirm the drill is calibrated to the proper seeding depth and seeding rate. Also check that disk openers are equipped to cut forage residue and create a sufficient opening in the soil.

“Even though we are beyond the date for optimum spring seeding, it is worth the risk to seed until mid-July this year because of the existing good soil moisture levels,” the authors assert. “If that timing is not feasible, then plan on harvesting or mowing the existing crop at about 60 days following planting, and seed immediately after.”